1. Field of the Invention
The present invention relates to a fuel cell system, and more particularly, to a fuel cell system with a high energy density and a compact size.
2. Discussion of the Background
A fuel cell is an energy generating system in which energy from a chemical is reaction is directly converted to electrical energy. The chemical reaction may be between hydrogen and oxygen, or between hydrogen contained in a hydrocarbon-based material such as methanol, ethanol, or natural gas, and oxygen. Fuel cells can be categorized as phosphoric acid type fuel cells, molten carbonate type fuel cells, solid oxide type fuel cells, polymer electrolyte membrane fuel cells, alkali type fuel cells, for example, according to the type of electrolyte that is used as a fuel. Most of these fuel cells operate upon the same principle, but may have different fuels, different operating temperatures, different catalysts, or different electrolytes.
Among these fuel cells, the polymer electrolyte membrane fuel cell (PEMFC) may have better output properties, lower operating temperature, shorter start-up time, and a quicker response than other fuel cells. Due to these advantages, the PEMFC may be used for a wide range of applications, including a portable power source for cars, an individual power source for homes or public buildings, and a small power source for electronic devices.
A direct methanol fuel cell (DMFC), which is a type of PEMFC, uses an aqueous methanol solution as a fuel. The DMFC can operate at room temperature and can be easily reduced in size and sealed so that it can be used as an energy-supplying source for various applications, such as clean electric vehicles, domestic energy generating systems, mobile communications equipment, medical equipment, military equipment, space business equipment, and portable electronics, for example.
The electric power generated from the DMFC is dependent upon the rate of the reaction occurring in an anode and a cathode. Specifically, in the anode, one molecule of methanol reacts with one molecule of water so that the methanol is oxidized. Carbon dioxide and six electrons are generated from the reaction, as illustrated in Reaction Scheme 1.CH3OH+H2O→CO2+6H++6e−  [Reaction Scheme 1]In the anode reaction shown in Reaction Scheme 1, the stoichiometry of the methanol to water is 1:1. Therefore, to continuously generate the anode reaction shown in Reaction Scheme 1, a DMFC should be constantly supplied with reactants in the proper ratio. However, in reality, more water is supplied than that amount required to completely oxidize the methanol. When the methanol is incompletely oxidized, reactions shown in Reaction Scheme 2 or Reaction Scheme 3 may occur, thus reducing energy generating efficiency.CH3OH+H2O→HCOOH+4H++4e−  [Reaction Scheme 2]CH3OH→HCHO+2H++2e−  [Reaction Scheme 3]
Methods of supplying a fuel to a DMFC may include an active supplying method and a passive supplying method. The active supplying method requires an external supplying unit that transports fuel to the DMFC under pressure. The passive supplying method includes supplying fuel without such pressure transport equipment.
In the active supplying method, the concentration of a reacting fluid supplied to an anode is appropriately maintained by supplying pure methanol or a high-concentration is methanol to a recirculation loop, which collects water generated by the reaction in a cathode and supplies the collected water to the anode. An advantage of the active supplying method is that the energy density of the entire system increases by using a methanol cartridge. A disadvantage of the active supplying method, however, is that the system is complex, the requirement for additional devices results in an increase in size of the DMFC, and additional power may be consumed since an external supplying system consumes energy for its operation. More particularly, since fuel cells are becoming smaller, the active supplying method has a disadvantage in that the necessary power-supplying source is also required to become smaller.
Conversely, a significant advantage of the passive supplying method is that it is a simple system. However, one disadvantage to the passive supplying method is that a fuel cartridge may contain water as well as the methanol fuel. Therefore, the volume of this fuel cartridge may increase the total volume of the fuel cell.
This problem has been addressed by supplying water generated in a cathode using the passive supplying method, an example of which is disclosed in U.S. Patent Application Publication No. 2004-209136. There, a hydrophobic microporous layer may be formed in a cathode of a membrane-electrode assembly, and water generated in the cathode can be transported to the anode by hydrostatic pressure.
However, in such a system, a unit cell formed of multiple layers may be damaged by the hydrostatic pressure. In addition, U.S. Patent Application Publication No. 2004-209136 does not teach a stable supply and dilution of methanol used as fuel.